This invention relates to an electronic sound processing system, and more specifically to an electronic sound processing system which executes a looping process on an input sound waveform.
A looping method or process performed on input sound or music waveforms by an electronic sound processing device is typically conducted by cutting out a predetermined cycle of a sound waveform and repeating the cycle thereby obtaining a looped sound waveform.
In prior art electronic sound processing devices such as a sampler, such a looping process is conducted by the following steps. First, approximately one cycle is cut out from a sound waveform shown on a display. Subsequently, connection points at which the cycle is expected to be smoothly looped or repeated are eye-estimated at the beginning and ending region of the cycle. The cycle is then repeated or looped at the connection points to become a looped waveform. In case the looped waveform is not smooth enough, the connection points are finely adjusted. The looped waveform thus obtaining goes through a low-pass filter (low-frequency passing filter) upon sound emission, thereby making the connection points having high frequency inconspicuous.
However, in the looping method adopted in the prior art sampler, connection points at which a cut out cycle of a waveform is smoothly repeated are not easily found, and thus a smooth looped waveform is rarely obtained. Therefore, a low-pass filter is necessary for removing the high frequency generated sound at the connection points at the time of sound emission, even if such a filter adds to the cost. Furthermore, since a smooth waveform is rarely obtained, the emitted tone quality of the looped sound waveform is not improved.
Japanese Published Examined Patent Application Nos. 61-39680 and 57-23278 disclose another type of electronic sound processing system such as an electronic piano and a synthesizer, which stores one cycle of a waveform to be looped in a ROM. In this electronic sound processing system, a looping process is conducted by the steps illustrated in FIGS. 5A through 5D. First, approximately one cycle T2 of a waveform shown in FIG. 5A is cut. Subsequently, as illustrated in FIG. 5B, a discrete Fourier transform is applied to the cycle T2. The discrete Fourier transform-applied waveform is then sampled at each angular frequency .theta..sub.0. FIG. 5C shows sampling points at which the waveform is sampled. Finally, angular frequencies .theta..sub.0 through n.theta..sub.0 and sampled values A1 through An at the sampling points 1 through n, respectively, are converted into sine waves. The obtained sine waves are then summed up according to the following formula (1): ##EQU1## where k.theta..sub.0 and Ak are the angular frequency and the value at the sampling point k, respectively. FIG. 5D shows a resulting one cycle T3 of the looped waveform. Since the frequencies of the summed sine waves are whole numbers times the angular frequency .theta..sub.0, the cycle T3 can be smoothly repeated at the connection points.
However, the looping process and especially the sampling step of the process in the latter type of the electronic sound processing system requires special equipment and complicated techniques, even though a smooth looped waveform is obtained. Therefore, the latter electronic sound processing system can be operated only when a special apparatus for sampling sound waveforms and summing sine waves is provided. Even if such an apparatus happens to be available, the looping process is still complicated and bothersome.